In order to improve fuel economy, a fuel cut-off control for stopping fuel supply to an engine during running, and a coasting control for coasting the vehicle by disconnecting the engine from a drive line during running have been employed in recent years. Specifically, the fuel cut-off is carried out to stop fuel supply to the engine given that an accelerator is returned to close an accelerator valve completely, and that an engine speed is higher than an idling speed so that a vehicle speed is higher than a predetermined speed. Under the fuel cut-off control, although the combustion of fuel in the engine is stopped, the engine is still rotating by a torque from the drive wheels. Therefore, a braking torque resulting from a pumping loss of the engine and a friction torque is applied to the drive wheels. Consequently, an engine braking force is applied to the vehicle.
For example, the coasting control is carried out when the accelerator is returned during running to coast the vehicle without rotating the engine passively by bringing the clutch into disengagement disposed between the engine and the drive wheels. Therefore, under the coasting control, the engine braking force will not be applied to the vehicle so that the vehicle is allowed to coast utilizing an inertia energy.
The coasting control may be carried out not only by stopping the fuel supply to the engine but also without stopping the fuel supply to the engine while lowering the engine speed close to the idling speed. In case of carrying out the coasting control while stopping the engine, the fuel will not be consumed during coasting the vehicle so that the fuel economy can be improved significantly. By contrast, in case of carrying out the coasting control without stopping the engine, the fuel economy will not be improved as the case of stopping the engine. In this case, however, it is not necessary to arrange additional devices for insuring a required oil pressure under cessation of the engine such as an electric oil pump, a hydraulic accumulator etc. Therefore, the coasting control can be carried out easily without requiring additional modification of the vehicle and additional equipment. In the following explanation, the coasting control without stopping the engine will be called the “neutral coasting control” or the “N coasting control”.
For example, Japanese Patent Laid-Open No. 2010-24773 describes a control for the neutral coasting. The coasting controller taught by Japanese Patent Laid-Open No. 2010-24773 is configured to bring the clutch disposed between the engine and the driving wheels into disengagement when the engine is not involved in propelling the vehicle, and to allow the vehicle by idling the engine. Specifically, the coasting controller is configured to commence the coasting control upon satisfaction of a starting condition based on an accelerator opening and a speed of a driven side of the clutch, and to terminate the coasting control upon satisfaction of a terminating condition based on the accelerator opening and the speed of a driven side of the clutch. In addition, the coasting controller is further configured to terminate the coasting control regardless of a satisfaction of the terminating condition if a difference between the vehicle speed at the commencement of the coasting control and the current vehicle speed exceeds a predetermined threshold value during execution of the coasting control.
Japanese Patent Laid-Open No. 2008-24010 describes a control device for hybrid vehicle configured to control a drive force and a regenerative braking force applied to the drive wheels from a motor-generator so as to achieve a target vehicle speed. According to the teachings of Japanese Patent Laid-Open No. 2008-24010, a limit value of the regenerative braking force of the motor-generator is determined on the basis of a deviation between the target vehicle speed and the current vehicle speed.
Japanese Patent Laid-Open No. 11-32404 describes a control equipment applied to an electric vehicle in which a regenerative braking force similar to an engine braking is established under the condition where both accelerator pedal and brake pedal are returned. According the teachings of Japanese Patent Laid-Open No. 11-32404, the regenerative braking force is changed in a manner such that an actual deceleration caused by the regenerative braking force is changed to achieve a target deceleration.
As described, the coasting controller taught by Japanese Patent Laid-Open No. 2010-24773 is configured to terminate the coasting control regardless of a satisfaction of the normal terminating condition if the vehicle speed is changed from that at the commencement of the coasting control and exceeds a predetermined threshold value during execution of the coasting control. Thus, according to the teachings of Japanese Patent Laid-Open No. 2010-24773, the coasting control can be terminated before the vehicle speed is changed to a certain extent during execution of the coasting control. Therefore, even if the vehicle speed is changed by a change in a road gradient, the vehicle speed will not be increased or decreased while coasting to exceed the threshold value so that a delay in acceleration and deceleration of the vehicle can be prevented.
However, if a timing to terminate the coasting control is determined based on a change in the vehicle speed from that at the commencement of the coasting control as taught by Japanese Patent Laid-Open No. 2010-24773, the coasting control may not be terminated at an appropriate timing. For example, if the coasting control is commenced when the vehicle runs on a flat road or climbs a gentle slope and then the vehicle runs down a steep slope, the vehicle is decelerated gently in the beginning and then accelerated significantly. That is, if the change in the deceleration of the vehicle does not exceed the threshold value in the beginning of the coasting control, the change in the vehicle speed will not be calculated until a point when the vehicle speed once decelerated in the beginning of the coasting control is raised again to the speed at the commencement of the coasting control, and the coasting control is continued until the change in the vehicle speed exceeds the threshold value. That is, a change in the vehicle speed increased from the lowest speed during deceleration is a net change in the vehicle speed of this case.
Thus, according to the coasting control taught by Japanese Patent Laid-Open No. 2010-24773, a net change in the increasing vehicle speed may be increased. Therefore, even if the acceleration felt by a driver or a passenger is increased by such increment in the net change in the increasing vehicle speed, the coasting control would be continued without applying a braking force to the vehicle for long period of time. For this reason, the driver and a passenger(s) may feel uncomfortable feeling or fear.